Previously, nitrogen rejection from natural gas was confined to a naturally occurring nitrogen content, thus an essentially constant feed composition. Recent methods of tertiary oil recovery utilizing nitrogen injection/rejection concepts, however, necessitate nitrogen rejection units (NRU) that can process a feed gas stream of a widely varying composition because the associated gas from the well becomes diluted by increasing amounts of injected nitrogen as the project continues. In order to sell this gas, nitrogen must be removed since it reduces the gas heating value. These nitrogen rejection processes typically use conventional heat exchangers to effect cooling of the natural gas feed stream.
Countercurrent heat exchange is commonly used in cryogenic processes because it is relatively more energy efficient than crossflow heat exchange. Heat exchangers of the plate-fin variety which are typically used in these processes can be configured in either a "cold-end up" or a "cold-end down" arrangement. When two-phase heat exchange, i.e. partial condensation, is effected one approach is to use the cold-end up arrangement because "pool boiling" may occur in a cold-end down arrangement when one of the refrigerant streams comprises many components. Pool boiling degrades the heat transfer performance of the heat exchanger. Therefore a cold-end up arrangement is preferred. The design of such cold-end up exchangers must insure that at all points in the exchanger, the velocity of the vapor phase is high enough to carry along the liquid phase and to avoid internal recirculation, i.e. liquid backmixing which degrades the heat transfer performance of the exchanger.
However, in certain processes, such typical cold-end up heat exchangers are not adequate. There are particular problems in heat exchange situations associated with cryogenic plants for purifying natural gas streams having a variable nitrogen content. One such application in a nitrogen rejection process for which conventional heat exchange technology is inadequate involves a natural gas feed stream which must be totally condensed at one feed composition in the early years, but which must only be partially condensed in the later years when the nitrogen content in the natural gas feed stream is much higher. As the nitrogen content gradually increases over the years, the cooled natural gas feed stream proceeds from a totally condensed stream to a two-phase cooled stream in which the fraction of the vapor phase increases with time. In such nitrogen rejection processes there is no vapor to carry over the liquid in the early years, so that the use of a conventional cold-end up heat exchanger is problematical.
A worker of ordinary skill in the art of cryogenic processes can choose from a host of heat exchangers such as, for example, helically wound coil exchangers, shell and tube exchangers, plate-exchangers and others.
Illustrative of the numerous patents showing heat exchangers having a serpentine pathway for at least one fluid passing in a heat transfer relationship with another fluid are U.S. Pat. Nos. 2,869,835; 3,225,824; 3,397,460; 3,731,736; 3,907,032 and 4,282,927. None of these patents disclose the use of a serpentine heat exchanger to solve the problem of liquid backmixing associated with heat exchangers for cooling a natural gas feed stream having a variable content in a nitrogen rejection scheme.
U.S. Pat. No. 2,940,271 discloses the use of tube heat exchangers in a process scheme for the separation of nitrogen from natural gas. No mention is made of the problems associated with cooling a multicomponent variable content gas stream.
U.S. Pat. No. 4,128,410 discloses a gas treating unit that uses external refrigeration to cool a high pressure natural gas stream by means of a serpentine, cold-end down heat exchanger. Since the refrigerant extracts heat from the natural gas stream as the refrigerant courses through the serpentine pathway in the heat exchanger, there is no problem with a two-phase upward condensing circuit.
U.S. Pat. No. 4,201,263 discloses an evaporator for boiling refrigerant in order to cool flowing water or other liquids. The evaporator uses a sinuous path consisting of multiple passes on the water side of the exchanger, in which each successive pass has less area, so that the velocity of the water is increased from the first pass to the last pass.
Serpentine heat exchangers have also been used in air separation processes as a single phase subcooler, that is for cooling a liquid stream to a lower temperature without backmixing due to density differences. Another application involves supercritical nitrogen feed cooling in a nitrogen wash plant over a region of substantial change in fluid density.